Lab notebook: the great hammerhead (Sphyrna mokarran)

Lab 1

Specific objectives:

  • Document the external anatomy of the specimen
  • Identify the specimen using taxonomic features

The great hammerhead (Sphyrna mokarran) is from the family Sphyrnidae of the “ground sharks” or Carcharhiniformes [1] (Fig. 1).

Figure 1 External morphology of the great hammerhead shark, (Sphyrna mokarran)

The sheer size of S. mokarran hints at its predatory nature. Its serrated, pointed teeth are excellent for tearing apart flesh from prey like fish and rays, or perhaps crushing shells of crustaceans too [2], [3]. The ampullae of Lorenzini, located on the underside of its snout, are electroreceptors in the form of a network of mucus-filled pores helping in prey detection [4]. Its head’s unique “hammer” shape is called a cephalofoil, a unique evolution to enhance the shark’s vision. The position of the eyes on the ends of the cephalofoil allow the shark to always see above and below itself [5]. The cephalofoil also increases surface area for distribution of the ampullae of Lorenzini, thereby allowing the shark to locate prey more effectively [6]. The cephalofoil acts as a lifting surface, similar to the hydrofoil of a boat, and assists with sharp turns to attack prey [3], [7]. The two nares under the snout act as nostrils where water can pass through, allowing the shark to “smell” and assist with prey detection. The shape of the head means the nares are located further apart which may enhance olfaction.

The two dorsal, pelvic, and anal fins act as vertical stabilisers (in males, the pelvic fin is modified into a reproductive organ known as a clasper). The two pectoral fins help to steer while swimming and, like a plane, provide lift in the water. The caudal fin provides thrust, and the caudal fin of S. mokarran has a smaller lower lobe than the upper lobe, indicating that it may spend much time swimming close to the seabed and that speed is not as essential for this species. S. mokarran likely has carangiform or thunniform propulsion because of the large, muscular tail.

The colouration of S. mokarran is known as countershading, whereby the top surface is darker than the lighter underside. This means that the shark is camouflaged from prey above it (looking down to the deep, dark ocean) and from prey below it (looking up at the light, sunlight, surface waters). This will help the shark ambush its prey effectively and, for younger/smaller individuals, avoid detection from carnivores looking for a hammerhead-shaped snack. Its skin is comprised of dermal denticles: rough, teeth-like scales that would help reduce swimming-induced drag and protect them from predators and parasites. The lateral line extends the length of the shark sides and helps orient the shark to movement and sound. As a sensory organ, it works in conjunction with the ampullae of Lorenzini to assist in prey detection.

Lab 2

Specific objective:

  • Describe the functions of its organs as it relates to the ecology/biology of the specimen

Sharks lack an operculum and instead have gill slits that act as openings to the gills [8], allowing seawater to flow over them and oxygen to be extracted for respiration. From viewing photographs of S. mokarran, it does not appear to have a spiracle, like other sharks do (except requiem sharks). The spiracle is a small hole behind the eye that opens to the buccal cavity, assisting sharks to take in water over their gills whilst stationary, suitable for sharks that spend much time on the seafloor [9]. The lack of a spiracle coincides with the pelagic nature of S. mokarran and infers that they must be ram ventilators who must continuously swim forward to encourage water flow over the gill filaments through the mouth or gill slits [9].

Elasmobranchs are unique in that the morphological structure of their jaw means that it is suspended by a musculoskeletal sling [10]. S. mokarran are even more unique in that their head is dorsoventrally compressed and laterally expanded to form the cephalofoil. Because the evolution of the cephalofoil is such a drastic, morphological difference among Carcharhiniforms, trade-offs have been made to the pharyngeal apparatus (as well as other systems that occupy the head) [11]. Additionally, the teeth are triangular and serrated (Fig. 2).

One observation [7] of S. mokarran predation on a stingray saw the shark use its cephalofoil to pin the ray to the seafloor on two occasions, each time taking a bite from the wings of the ray, rendering it incapacitated. The hammerhead was then able to consume the immobile ray easily.

Figure 2 Teeth in dried jaws of great hammerhead shark, Sphyrna mokarran. Doug Perrine. (2013). received from https://www.alamy.com/stock-photo-teeth-in-dried-jaws-of-great-hammerhead-shark-sphyrna-mokarran-on-130546748.html on 29 September 2021.

There is limited research on S. mokarran digestion specifically, so it is helpful to infer similar characteristics from other species with a similar diet. The digestive tract begins at the mouth, where a strong jaw and sharp, replaceable teeth can rip flesh or crush prey into swallowable sizes. This is where the first type of digestion takes place – mechanical.

Food travels through the oesophagus from the mouth, where striated muscles and secreted mucous assist food into the stomach [12]. The stomach of most sharks is J-shaped [13], although there are some exceptions. The bonnethead shark (Sphyrna tiburo) exhibits a straight (I-shaped) stomach [14]. Because S. tiburo is from the same genus as S. mokarran, it may seem wise to assume that S. mokarran has an I-shaped stomach too, but S. tiburo is omnivorous and digests seagrass, whereas S. mokarran is strictly carnivorous. Like other vertebrates, the cells on the stomach walls of an elasmobranch secrete mucous to protect it from the acidic gastric juices that biochemically digest food stored in the stomach [12]. Some sharks are known to undergo gastric evacuation, whereby undigestible contents of the stomach are regurgitated out of the mouth [11], [12], although it is unknown is S. mokarran can do this. Furthermore, some elasmobranch species can regulate gastric acid secretion, likely in times of fasting due to low prey availability [15].

Nutrients are transported to the intestine from the stomach, which is relatively shorter than most vertebrate intestines. Leigh et al. (2017) recommend separating elasmobranch intestines into three sections: proximal, spiral, and distal. The proximal region gives way to the spiral region or spiral valve. The anatomy of the spiral valve varies among species but can have between 2–50 turns and is thought to increase surface area for absorption of nutrients and/or slow the rate at which food travels through the intestine, therefore, increasing time available for digestion [14], [16]. Furthermore, the spiral valve ensures that oversized items (e.g., bones) cannot pass through their lower intestine and allows them to be sufficiently broken down first or regurgitated.

After the spiral valve is the distal intestine, characterised by thicker and more muscular walls to accommodate the accumulation of faeces [12]. As pressure increases on the rectal walls, nerve impulses are sent to the brain for muscles to relax, and faeces are passed through the cloaca, which serves as the anus as well as the genitals and urinary duct [12]. Another species from the same genus as S. mokarran, the scalloped hammerhead (Sphyrna lewini), demonstrates an increased gastric evacuation rate with increased meal size [17], so this may be the case for S. mokarran as well. The amount of speculation I’m having to undergo for S. mokarran based off closely related species is evidence that S. mokarran needs more research and investigation to better understand it.

The small, relative size of the elasmobranch digestive tract may make room for the large liver. Baldridge (1970) found that the liver accounted for 3.83% and 9.5% of total body weight in two S. mokarran individuals (imagine a 100kg man having a 3.8 kg or 9.5 kg liver!!). Elasmobranchs do not have swim bladders like bony fish and instead rely on the liver, which is saturated in oil, to maintain buoyancy [19].  The liver contains lightweight oils, increasing its buoyancy and, along with its fins, gives it the lift it needs to prevent sinking.

Overall, not much is known about the reproductive biology of S. mokarran. Male specimens of S. mokarran have two claspers inside each pelvic fin [20] that deposit sperm into the female’s cloaca [21]. Based on evidence from other shark species, it is apparent that females can store sperm in their shell gland for up to 16 months [21]. S. mokarran are viviparous (birthing live young), and the ova has a yolk sac that, once depleted, turns into a structure similar to a placenta [1], [21]. They usually litter between 6–42 pups after a gestation of ~11 months [1], although one female was known to have littered a record 55 pups [22].

Lab 3

Specific objective:

  • Provide information about the age and growth of the specimen

The maximum reported age for S. mokarran is 30 years [1], although one specimen was estimated to be around 40–50 years old [22].

S. mokarran are the largest hammerhead species, and a male can reach a maximum total length (TL) of up to 610 cm, although typically, they will average 370 cm TL [1]. Due to their lack of otoliths, cartilaginous fish are aged by counting banding patterns on their sagittal vertebrae (Fig. 3). The von Bertalanffy growth function (VBGF) (Fig. 4) shows that, at birth, S. mokarran is around 50-70 cm.

Figure 3 Taken from [23]: “Sagittal vertebral section from a 4-year-old great hammerhead, Sphyrna mokarran, illustrating the banding pattern and annuli used to assign age. Scale bar = 2 mm.”
Figure 4 Taken from [23]: “The best fit von Bertalanffy growth model for male and female great hammerhead sharks, Sphyrna mokarran, collected in the northwestern Atlantic Ocean and the eastern Gulf of Mexico.”

The VBGF (Fig. 4) shows that males grow faster than females but reach a smaller asymptotic size than females. This growth is likely due to different energy requirements for the sexes for somatic growth and reproductive development [23].

S. mokarran reaches sexual maturity between 210–300 cm total length, with males tending to reach maturity at a smaller size than females [1].

Lab 4

Specific objective:

  • Provide information about the reproductive dynamics and life history of your chosen specimen

The embryonic sex ratio of S. mokarran is close to 1:1 [24]. It is gonochoric, with no interesting or outstanding sexual dynamics to note [1].

Rigby et al. (2019) describe S. mokarran as aplacental viviparous, whereas Froese and Pauly (2021) describe the species as viviparous with a yolk-sac placenta. Either way, they birth live young that have hatched from an egg in-utero.

There is not much research into the spawning behaviour of S. mokarran, which is perhaps reflective of their naturally elusive lifestyle. Stevens and Lyle (1989) note mating scars on females, which indicates, like many shark species, the courtship process can be seemingly violent with the male holding onto the females with his teeth during copulation.

S. lewini were observed in a large group off the Galapagos Islands and were thought to be amidst a courtship ritual where the largest females dominated the centre of the group, and the males attempted to access them to mate [26]. This could provide insight into the mating rituals of S. mokarran, although S. mokarran populations are substantially smaller than S. lewini, and they have yet to be observed in such large numbers.

One account of mating S. mokarran in the Bahamas reported two individuals ascending in 21m of water as they spiraled around one another and copulated at the surface [27].

S. mokarran birth between 6–42 pups every two years [25]. Their parental mode is not well researched. Many elasmobranchs offer no maternal care once the pup is born, so it can be assumed that this is the same for S. mokarran. However, a study on the scalloped hammerhead (S. lewini) and the Carolina hammerhead (S. gilberti) illustrated that neonatal hammerheads are likely to rely on maternal provisioning in the first few weeks after birth [28]. Therefore, an increased maternal investment may be a part of the life history strategy of S. mokarran. Again, further research is crucial to understand their reproductive and life histories further.

There is regional variation in the size and age range of S. mokarran sexual maturity. As before mentioned, this species reaches sexual maturity between 210–300 cm total length, with males maturing from 225–269 cm and females maturing from 210–300 cm [1], [25]. Age-at-maturity for females is estimated to be 5.5–8.3 years in Atlantic and Pacific populations [25], [29].

S. mokarran eggs hatch in-utero and embryonic individuals spend 11 months in their mother’s uterus, and newborns are around 50–70 cm total length and are then known as pups [25], [29]. At birth, they resemble S. mokarran adults in external morphology (Fig. 5). They grow rapidly until ten years of age, where their growth rate reduces [23], likely because they have reached sexual maturity and fitness rather than size becomes more critical for courtship and survival.

Figure 5 Neonatal great hammerhead, Sphyrna mokarran, pups. By Apex Predators Program, NOAA/NEFSC – http://nefsc.noaa.gov/rcb/photogallery/sharks/sharks.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=20004725

[1]      R. Froese and D. Pauly, “Sphyrna mokarran,” Fishbase, 2021. https://www.fishbase.se/summary/Sphyrna-mokarran.html (accessed Sep. 21, 2021).

[2]      V. Raoult, M. K. Broadhurst, V. M. Peddemors, J. E. Williamson, and T. F. Gaston, “Resource use of great hammerhead sharks (Sphyrna mokarran) off eastern Australia,” J. Fish Biol., vol. 95, no. 6, pp. 1430–1440, 2019, doi: 10.1111/jfb.14160.

[3]      D. D. Chapman and S. H. Gruber, “A further observation of the prey-handling behavior of the great hammerhead shark, Sphyrna mokarran: Predation upon the spotted eagle ray, Aetobatus narinari,” Bull. Mar. Sci., vol. 70, no. 3, pp. 947–952, 2002.

[4]      E. E. Josberger et al., “Proton conductivity in ampullae of Lorenzini jelly,” Sci. Adv., vol. 2, no. 5, pp. 1–7, 2016, doi: 10.1126/sciadv.1600112.

[5]      K. R. Mara, “Evolution of the Hammerhead Cephalofoil: Shape Change, Space Utilization, and Feeding Biomechanics in Hammerhead Sharks (Sphyrnidae),” University of South Florida, 2010.

[6]      S. M. Kajiura, J. B. Forni, and A. P. Summers, “Olfactory morphology of carcharhinid and sphyrnid sharks: Does the cephalofoil confer a sensory advantage?,” J. Morphol., vol. 264, no. 3, pp. 253–263, 2005, doi: 10.1002/jmor.10208.

[7]      W. R. Strong, F. F. Snelson, and S. H. Gruber, “Hammerhead Shark Predation on Stingrays: An Observation of Prey Handling by Sphyrna mokarran,” Copeia, vol. 1990, no. 3, p. 836, 1990, doi: 10.2307/1446449.

[8]      W. J. Vanderwright, J. S. Bigman, C. F. Elcombe, and N. K. Dulvy, “Gill slits provide a window into the respiratory physiology of sharks,” Conserv. Physiol., vol. 8, no. 1, pp. 1–10, 2020, doi: 10.1093/conphys/coaa102.

[9]      J. L. Dolce and C. D. Wilga, “Evolutionary and Ecological Relationships of Gill Slit Morphology in Extant Sharks,” Bull. Museum Comp. Zool., vol. 161, no. 3, p. 79, 2013, doi: 10.3099/mcz2.1.

[10]    P. J. Motta, “Prey Capture Behavior and Feeding Mechanics of Elasmobranchs,” in Biology of Sharks and Their Relatives, J. C. Carrier, J. A. Musick, and M. R. Heithaus, Eds. Boca Raton: CRC Press, 2004, pp. 165–202.

[11]    J. M. Brunnschweiler, P. L. R. Andrews, E. J. Southall, M. Pickering, and D. W. Sims, “Rapid voluntary stomach eversion in a free-living shark,” J. Mar. Biol. Assoc. United Kingdom, vol. 85, no. 5, pp. 1141–1144, 2005, doi: 10.1017/S0025315405012208.

[12]    S. C. Leigh, Y. Papastamatiou, and D. P. German, “The nutritional physiology of sharks,” Rev. Fish Biol. Fish., vol. 27, no. 3, pp. 561–585, 2017, doi: 10.1007/s11160-017-9481-2.

[13]    W. C. Hamlett, Sharks, skates, and rays: the biology of shark fishes. Baltimore: The Johns Hopkins University Press, 1999.

[14]    P. Jhaveri, Y. P. Papastamatiou, and D. P. German, “Comparative Biochemistry and Physiology , Part A Digestive enzyme activities in the guts of bonnethead sharks ( Sphyrna tiburo ) provide insight into their digestive strategy and evidence for microbial digestion in their hindguts,” Comp. Biochem. Physiol. Part A, vol. 189, pp. 76–83, 2015, doi: 10.1016/j.cbpa.2015.07.013.

[15]    R. D. Day, I. R. Tibbetts, and S. M. Secor, “Physiological responses to short-term fasting among herbivorous, omnivorous, and carnivorous fishes,” J. Comp. Physiol. B Biochem. Syst. Environ. Physiol., vol. 184, no. 4, pp. 497–512, 2014, doi: 10.1007/s00360-014-0813-4.

[16]    C. Bucking, “Feeding and Digestion in Elasmobranchs: Tying Diet and Physiology Together,” in Fish Physiology, vol. 34, Academic Press, 2015, pp. 347–394.

[17]    A. Bush and K. Holland, “Food limitation in a nursery area estimates of daily ration in juvenile scalloped hammerheads,” J. Exp. Biol. Ecol., vol. 278, pp. 157–178, 2002.

[18]    H. D. Baldridge, “Sinking Factors and Average Densities of Florida Sharks as Functions of Liver Buoyancy Published by : American Society of Ichthyologists and Herpetologists ( ASIH ) Stable URL : http://www.jstor.org/stable/1442317 REFERENCES Linked references are availabl,” Copeia, vol. 1970, no. 4, pp. 744–754, 1970.

[19]    M. Aidan, “Does Liver Size Limit Shark Body Size?,” Biology of Sharks and Rays, 2021. http://www.elasmo-research.org/education/topics/p_liver_size.htm (accessed Sep. 27, 2021).

[20]    M. Aidan, “Why Do Sharks Have Two Penises?,” Biology of Sharks and Rays, 2021. http://www.elasmo-research.org/education/topics/lh_2penises.htm (accessed Sep. 27, 2021).

[21]    M. Aidan, “From Here to Maternity,” Biology of Sharks and Rays, 2021. http://www.elasmo-research.org/education/topics/lh_maternity.htm (accessed Sep. 27, 2021).

[22]    “Record Hammerhead Pregnant With 55 Pups,” Discovery Channel, 2006. https://web.archive.org/web/20110622001318/http://dsc.discovery.com/news/2006/07/24/hammerhead_ani.html?category=earth&guid=20060724100030 (accessed Sep. 27, 2021).

[23]    A. N. Piercy, J. K. Carlson, and M. S. Passerotti, “Age and growth of the great hammerhead shark, Sphyrna mokarran, in the north-western Atlantic Ocean and Gulf of Mexico,” Mar. Freshw. Res., vol. 61, no. 9, pp. 992–998, 2010, doi: 10.1071/MF09227.

[24]    J. D. Stevens and J. M. Lyle, “Biology of three hammerhead sharks (Eusphyra blochii, sphyrna mokarran and s. lewini) from northern australia,” Mar. Freshw. Res., vol. 40, no. 2, pp. 46–129, 1989, doi: 10.1071/MF9890129.

[25]    C. L. Rigby et al., “Sphyrna mokarran, Great Hammerhead,” IUCN Red List Threat. Species, vol. e.T39386A2, p. 16, 2019, [Online]. Available: http://dx.doi.org/10.2305/IUCN.UK.2007.RLTS.T39386A10191938.en.

[26]    BBC Earth, “Hammerhead Sharks’ Complex Mating Rituals | BBC Earth,” 2019. https://www.youtube.com/watch?v=KsWuJtQpgsw (accessed Oct. 06, 2021).

[27]    “Great hammerhead shark – Sphyrna mokarran,” Shark Research Institute, 2021. https://www.sharks.org/great-hammerhead-shark-sphyrna-mokarran (accessed Sep. 27, 2021).

[28]    K. Lyons et al., “Maternal provisioning gives young-of-the-year Hammerheads a head start in early life,” Mar. Biol., vol. 167, no. 11, pp. 1–13, 2020, doi: 10.1007/s00227-020-03766-y.

[29]    H. H. Hsu et al., “Biological aspects of juvenile great hammerhead sharks Sphyrna mokarran from the Arabian Gulf,” Mar. Freshw. Res., vol. 72, no. 1, pp. 110–117, 2020, doi: 10.1071/MF19368.

An introduction to the thresher shark

Introduction

The thresher shark is an all-encompassing term referring to three surviving species from the family Alopiidae: the pelagic thresher (Alopias pelagicus), the bigeye thresher (Alopias superciliosus), and the common thresher (Alopias vulpinus).

Pelagic thresher shark (Alopias pelagicus). By Thomas Alexander – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50280277
Bigeye thresher shark (Alopias superciliosus). By PIRO-NOAA Observer Program – http://ias.pifsc.noaa.gov/lds/obs_training/SharkThresherNew.pdf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534429
Comparison between the three thresher species. By FactZoo.com – https://www.factzoo.com/fish/pelagic-thresher-shark-longtail-smack.html

Morphology

Thresher jaws are small. They have clearly not evolved to attack prey like tuna and seals; if anything, they look like they should be eating small crabs or scavenging chunks of floating debris that someone else took the time to kill. They would seemingly struggle to catch a live fish because the long snout overhangs their tiny mouth so much. That’s where the notorious tail comes in. The thresher tail is designed to be used like a whip to strike and immobilise prey [1], [2]. Such force occurs with a single tail slap that gas is diffused out of the seawater and rises to the surface in bubbles [1]. Typically, pelagic sharks hunt and capture one fish at a time; this strategy enables the shark to catch on average 3 fish, and sometimes more, in one go.

A sequence of still images taken from an overhead tail-slap hunting event [1].

The easiest way to differentiate between the three species is the colouration. Common threshers are likely to be more grey and they lack any colouration above their pectoral fins; sometimes, they have white dots on the tips of their fins. Pelagic threshers have distinct colouration above their pectoral fins. The bigeye’s eye is visible from the top of the head and has characteristic groves above the eyes and gills.

A common thresher shark, identified by the lack of colouration above the pectoral fin and white dots on the tips of the pectoral and dorsal fins. By Paul E Ester at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19628266
A pelagic thresher shark, identified by the colouration above its pectoral fin. By NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_pelagic_thresher_sharks/obs_pelagic_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5553329
A bigeye thresher shark, distinguished by the birds-eye view of the large eyes and the lateral grooves. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534392
A bigeye thresher shark is distinguished by its large eyes and lateral groove above the eye and gills. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark4.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534388

Distribution

Threshers are pelagic, meaning they live in the deep, open ocean. The only thresher known to inhabit New Zealand waters is the largest of the trio, the common thresher [3]. The bigeye may also be found in NZ, but due to its habit of staying hundreds of metres below the ocean surface during the day, it is unlikely to be encountered if it is there at all [4].

Threats

The common and bigeye thresher are ‘Vulnerable’ to extinction under the IUCN Red List [3], [5], and the pelagic thresher is further threatened and classified as ‘Endangered’ [6]. Estimated global population reductions are as follows:

  • Common thresher: 30–49% over 3 generations (76.5 years)
  • Bigeye thresher: 30–49% over 3 generations (55.5 years)
  • Pelagic thresher: 50–79% over 3 generations (55.5 years)

All three species are globally targeted for their meat, fins, skin, and liver oil [7], the latter commonly used in modern cosmetics and supplements. All species are usually retained if accidentally caught on commercial vessels [8]–[11]. In one study [15] conducted in the Indian Ocean, 13 out of 19 bigeye threshers caught on a commercial vessel’s longline were dead upon retrieval. In the same study, in the Atlantic Ocean, 412 out of 849 bigeye threshers caught on longlines were dead. On the same ships were deaths of stingrays and manta rays as well as blue, shortfin mako, silky, and smooth hammerhead sharks. One of the largest hubs for shark fin trading globally is Hong Kong [9], and threshers accounted for up to 3% of the total fin mass in the past [8].

Threshers are also popular among recreational, big game anglers. Although a tag and release method is more commonly practised nowadays, there is also a risk of post-release mortality. With threshers likely to be hooked from their tail due to their hunting style, a study of common threshers found 78% of tail-hooked sharks died after release [12].

The New Zealand commercial fishery is not exempt from thresher shark landings. A recent report by Fisheries New Zealand [13] saw that in 5 years from the 2014/15 to 2018/19 commercial fishing seasons, 149,916 tonnes of common thresher shark were caught by the core deep-water fleet. With the average 5-metre common thresher shark usually weighing in at 230 kg, we can infer that in the 5 years, the deep-water fleet landed approximately 651 individual thresher sharks.

Thresher vs swordfish

In April 2020, a dead, 4.5 metre, female bigeye thresher was found beached on a Libyan coast [14]. A 30 cm swordfish (Xiphias gladius) rostrum was found embedded in its head. It is understood that the rostrum severely injured some of the shark’s nerves, arteries, and gill arches. It was concluded that the impalement was what led to the ultimate death of the shark.

A young swordfish (Xiphias gladius). By Michael Landress, CC BY-NC-ND 2.0https://www.flickr.com/photos/myfwc/26802909040
Taken from Jambura et al (2021) [14]. “Female bigeye thresher Alopias superciliosus (TL = 445 cm) stranded on the Libyan coast (Mediterranean Sea), with a swordfish Xiphias gladius rostrum embedded deep in the branchial chamber. Scale bars indicate 50 cm (b) and 10 cm (c and d). Photo (a and b) and video content (c and d) courtesy of Faraj Habrisha and Abdalhakim Ahmed Al sebaihe”.

[1]        S. P. Oliver, J. R. Turner, K. Gann, M. Silvosa, and T. D’Urban Jackson, “Thresher Sharks Use Tail-Slaps as a Hunting Strategy,” PLoS One, vol. 8, no. 7, 2013, doi: 10.1371/journal.pone.0067380.

[2]        S. A. Aalbers, D. Bernal, and C. A. Sepulveda, “The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus,” J. Fish Biol., vol. 76, no. 7, pp. 1863–1868, 2010, doi: 10.1111/j.1095-8649.2010.02616.x.

[3]        C. L. Rigby et al., “Alopias vulpinus,” IUCN Red List Threat. Species 2019, vol. e.T39339A2, 2019.

[4]        H. Nakano, H. Matsunaga, H. Okamoto, and M. Okazaki, “Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean,” Mar. Ecol. Prog. Ser., vol. 265, pp. 255–261, 2003, doi: 10.3354/meps265255.

[5]        C. L. Rigby et al., “Alopias superciliosus,” IUCN Red List Threat. Species 2019, vol. e.T161696A, 2019.

[6]        C. L. Rigby et al., “Alopias pelagicus,” IUCN Red List Threat. Species 2019, vol. e.T161597A, 2019, doi: https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T161597A68607857.en.

[7]        R. W. Jabado et al., “The trade in sharks and their products in the United Arab Emirates,” Biol. Conserv., vol. 181, pp. 190–198, 2015, doi: 10.1016/j.biocon.2014.10.032.

[8]        S. C. Clarke, J. E. Magnussen, D. L. Abercrombie, M. K. McAllister, and M. S. Shivji, “Identification of shark species composition and proportion in the Hong Kong shark fin market based on molecular genetics and trade records,” Conserv. Biol., vol. 20, no. 1, pp. 201–211, 2006, doi: 10.1111/j.1523-1739.2005.00247.x.

[9]        A. T. Fields et al., “Species composition of the international shark fin trade assessed through a retail-market survey in Hong Kong,” Conserv. Biol., vol. 32, no. 2, pp. 376–389, 2018, doi: 10.1111/cobi.13043.

[10]      S. C. Clarke et al., “Global estimates of shark catches using trade records from commercial markets,” Ecol. Lett., vol. 9, no. 10, pp. 1115–1126, 2006, doi: 10.1111/j.1461-0248.2006.00968.x.

[11]      F. Dent and S. Clarke, “State of the global market for shark products,” Rome, Italy, 2015.

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[13]      Fisheries New Zealand, “Annual Review Report for Deepwater Fisheries for 2018/19,” 2020.

[14]      P. L. Jambura, J. Türtscher, J. Kriwet, and S. A. A. Al Mabruk, “Deadly interaction between a swordfish Xiphias gladius and a bigeye thresher shark Alopias superciliosus,” Ichthyol. Res., vol. 68, no. 2, pp. 317–321, 2021, doi: 10.1007/s10228-020-00787-x.

[15]      R. Coelho, J. Fernandez-Carvalho, P. G. Lino, and M. N. Santos, “An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean,” Aquat. Living Resour., vol. 25, no. 4, pp. 311–319, 2012, doi: 10.1051/alr/2012030.

Shortfin mako: the world’s fastest shark is speeding towards extinction

Introduction

The shortfin mako shark (Isurus oxyrinchus) is a pelagic, cartilaginous fish with a wide distribution range that covers most oceans and undergoes migrations that can be as large as 5,300km in just under 1.5 years (Barreto, de Farias, Andrade, Santana, & Lessa, 2016; Kohler, Turner, Hoey, Natanson, & Briggs, 2002). There is high demand for mako shark meat, and it is a prize game species in recreational fishing worldwide (Barreto et al., 2016).

As of 2019, I. oxyrinchus is classified as “Endangered” on the International Union for the Conservation of Nature’s (IUCN) Red List (Rigby et al., n.d.). Rigby et al. (n.d.) concluded that shortfin mako’s population trend is decreasing; there is an estimated decline everywhere except in the South Pacific and an overall estimated average reduction of 46.6% over 72-75 years. There is an estimated decline in biomass and abundance of 99.9% since the early 1800s, the main reason being overfishing (Ferretti et al., 2008).

Morphology

Shortfin mako circulatory systems utilise a heat-exchanging technique that raises their internal temperature above that of the surrounding environment (Carey, Teal, & Kanwisher, 1981; Kohler et al., 2002). They have streamlined bodies, and aerobic muscles centred closer to their rear, which aids in thunniform swimming and increases power (Donley, Sepulveda, Konstantinidis, Gemballa, & Shadwick, 2004; Wegner, Sepulveda, Olson, Hyndman, & Graham, 2010). Emery and Szczepanski (1986) concluded that the gill area of I. oxyrinchus is 2-3 times larger than other pelagic shark species, which could aid in the mako’s speed, agility, and ability to swim long distances. Shortfin mako habitat extends globally in tropical and temperate oceans and they can be found inshore in coastal areas or at least 500m down in oceanic zones (Kohler et al., 2002).

Life history

Sexual dimorphism is prevalent in I. oxyrinchus,with females often occurring larger than males, with males reaching a maximum size of 2.6m and females reaching a maximum size of 3.4m (Barreto et al., 2016; Cema & Lincandeo, 2009; Chan, 2001; Doño, Montealegre-Quijano, Domingo, & Kinas, 2014; Hsu, 2001; Natanson et al., 2006; Semba, Nakana, & Aoki, 2009). Shortfin mako sharks are oophagous and ovoviviparous (Kohler et al., 2002), and a study by Mollet et al. (2000) predicts a gestation period of 15-18 months, although a study by Duffy and Francis (2001) puts makos in New Zealand waters at a 21-month gestation period. They have a 3-year reproductive cycle (Mollet & Cailliet, 2002). Bishop, Francis, Duffy, and Montgomery (2006) concluded that New Zealand shortfin mako births are concentrated in spring and gave a theoretical birth date of 1 October, with the average length of the shark at birth to be 61cm. Francis and Duffy’s (2005) study on sexual maturity of New Zealand shortfin makos concluded that maturity occurs between 7-9 years for males and 19-21 years for females (Bishop et al., 2006). Bishop et al. (2006) found evidence of New Zealand shortfin makos living to 29 years, although this number is probably higher because there is a lower chance of catching older sharks which make up a small percentage of the overall population. The same study found the sharks grow quickly within their first year after birth; this growth rate rapidly reduces in the subsequent years to steadier growth. Late maturity, moderately long longevity, the estimated low natural mortality rate, and low annual fecundity causes low productivity in the species (Bishop et al., 2006; Mollet et al., 2000).

Diet

Stillwell and Kohler (1982) analysed the stomach contents of shortfin mako sharks and found evidence of bony fish, swordfish, and cephalopods. As shark body length increased, so did the average volume of food, indicating that as makos grow larger, they may switch to larger prey items (Kohler et al., 2002).

Mortality

Sharks have evolved for 400 million years (Donley et al., 2004), leading species such as I. oxyrinchus to be apex predators at the top of their food chain; they have no natural predators which results in low natural mortality. There is evidence of parasitic activity in shortfin makos, although it could not be determined if there was a negative effect on the shark (Borucinska & Hege, 1999). One example of anthropogenically influenced mortality is seen in 1966 when a longfin mako (Isurus paucus) died from a fishing hook retained in its flesh (Adams, Borucinska, Maillett, Whitburn, & Sander, 2015).

Ecology

Sharks are keystone species and have a strong effect on multiple ecosystems due to their predatory role and wide dispersal range (Feretti, Worm, Britten, Heithaus, & Lotze, 2010). I. oxyrinchus is considered to be a large shark with “strong, top-down forces” (Feretti et al., 2010, p1055). Therefore, their removal from an ecosystem is highly likely to drastically alter communities, induce trophic cascades, release mesopredators such as smaller sharks and rays, and consequently cause a decline in commercial fish stocks (Fig. 1).

Figure 1 “Documented ecosystem effects of fishing large sharks. Depicted are trophic (solid arrows) and behavioral (dotted arrows) interactions between humans, large and mesopredator elasmobranchs and their prey species. Block arrows represent overall population trends of the various functional groups. Regions in which particular interactions have been documented (see text) are indicated by letters (A, Australia; C, Caribbean; E, Europe; G, Gulf of Mexico; M, Mediterranean Sea; N, North American East Coast; P, Central Pacific; S, South Africa; W, North American West Coast).” (Feretti et al. 2010)

Threats

There is a high commercial demand for shortfin mako shark meat, and the species is exploited globally (Barreto et al., 2016). Peru is one of the biggest shark-fishing nations, and I. oxyrinchus is one of the top 2 most caught species with a rarely enforced catch size limit (Gonzalez-Pestana, Kouri, & Velez-Zuazo, 2016; Fischer, Erikstein, D’Offay, Guggisberg, & Barone, 2012). Brazilian fleets in the western and central South Atlantic exploit immature shortfin makos, specifically females (Barreto, 2016); it was rare for them to catch individuals greater than 2m. On a global scale, it is both a target and bycatch species in commercial and small-scale fisheries, including longline, gillnet, purse seine, trammel net, and trawls (Camhi, Pikitch, & Babcock, 2008; Rigby et al., 2019). There is likely underreporting of catch, and commercial post-release mortality from longlines alone is reported at 30-33% (Campana, Joyce, Fowler, & Showell, 2016; Rigby et al., 2019). Shortfin mako fins made up 1.2% of shark fin imported to Hong Kong in 2014 (Fields et al., 2017), and their skin, jaws, and liver oil are also used (Compagno, 2001). In New Zealand’s EEZ, I. oxyrinchus is a common bycatch species on tuna longlines and less commonly on pelagic longlines, trawls, and set nets (Bagley et al., 2000; Francis, 1998; Francis, Griggs, Baird, Murray, & Dean, 2000; Martinsohn & Muller, 1992). Since 1993, commercial catch averaged 60 tonnes annually, but observer reporting estimated 100-200 tonnes per year from the longline tuna fishery alone (Francis, 1998; Francis et al. 2000); the discrepancy between these numbers could be related to lack of accurate recording. From 1988-2015, the New Zealand tuna longline fishery total catch was comprised of 11.1% target species (southern bluefin tuna, bigeye tuna, and swordfish) and 88.9% bycatch (including albacore tuna, lancetfish, porbeagle shark, deepwater dogfish, dealfish, mako shark, moonfish, escolar, sunfish, and butterfly tuna) according to a report by Fisheries New Zealand [FNZ] (2018). Since October 2014, shark finning is illegal in New Zealand, yet over half of all makos caught by charter vessels in the 2014-15 tuna longline fishery year were kept for their flesh (FNZ, 2018). The low productivity of mako sharks causes them to be vulnerable to overfishing and makes it hard for the species to replace lost individuals. CITES (2019) reported that shortfin mako is in danger of population collapse due to the overfishing of most of the juveniles since the 1980s; this means that sharks dying of old age now will not be replaced with mature individuals leaving a 10-20-year gap.

The mining of precious metals increases the presence of toxic contaminants in the environment, especially in coastal environments, specifically mercury (Maz-Courrau et al., 2011). Sharks bioaccumulate mercury through their tissues and organs from the environment and through the food they eat; mercury travels up the trophic web and magnifies at each level, eventually accumulating in shark muscular tissue (Maz-Courrau et al., 2011). Maz-Courrau et al. (2011) found that 33% of shortfin mako sharks they studied off the Baja California coast had levels of mercury higher than was estimated to be fit for human consumption by Mexican Law and Watling et al. (1981) found mercury concentrations in mako sharks off the coast of Australia to be almost twice that. As Maz-Corrau et al. (2011) only sampled juveniles, it is likely that as sharks grow and eat more, more mercury is accumulated into their tissue over time. When people eat this contaminated tissue, it bioaccumulates up the food chain again and causes adverse health effects in humans.

Current/Proposed Management Actions

As of 2019, I. oxyrinchus officially met criteria to be listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II, which “includes species not necessarily threatened with extinction, but in which trade must be controlled in order to avoid utilisation incompatible with their survival” (CITES, 2019, p. 2).

Pressure from overfishing appears to be the most significant contributor to shortfin mako shark decline globally. Due to the widespread distribution of I. oxyrinchus and migratory nature, conservation effort and management is needed at both a local and international scale (Adams, Flores, Flores, Aarestrup, & Svendsen, 2016; Corrigan et al., 2018). Because of their “Endangered” status on the with a decreasing population trend, it is essential to increase their population trend and bring them to the next highest, more stable level on the Red List, “Vulnerable”, and take measures to ensure they do not go the other direction and become classified as “Critically Endangered”. The most effective way this can be achieved is through strict management of fisheries and the implementation of Marine Protected Areas (MPA) in habitats that are suitable for shortfin makos by means of international treaties (Birkmanis, Partridge, Simmons, Heupel, & Sequeira, 2020; Dulvy et al., 2008; Rigby et al., 2019).

Community Education & Awareness

Films, documentaries, and TV series are a great way of moving an audience and opening their eyes to what is going on in the world around them in an entertaining way. Saving Jaws by Ocean Ramsey, Shark Week by The Discovery Channel, and BBC’s Blue Planet, narrated by David Attenborough, are a few.

For the booklovers, don’t worry, we got you. Ocean Ramsey’s What You Should Know About Sharks: Shark Language, social behavior, human inter-actions, and life saving information educates readers on shark behaviours, how to swim with them safely, and debunks common shark myths. Sylvia A. Earle’s book The World is Blue: How Our Fate and the Ocean’s are One shows readers how every one of us literally breathes the ocean, and her other book, Sea Change: A Message of the Oceans, has been compared to Rachel Carson’s Silent Spring. For those who do not enjoy reading, Christian Vizl’s Silent Kingdom is a photography-based book that shows the raw beauty, power, and elegance of marine creatures, such as sharks, in stunning black and white photos.

NGO’s also play an essential role in community education and awareness. In New Zealand, groups such as Auckland Whale & Dolphin Safari and Whale Watch Kaikōura take their customers to visit sea life up close. SEA LIFE Kelly Tarlton’s Aquarium educates both adults and children alike and cares for sick and dying turtles.

Monitoring & Research

There is enough research on I. oxyrinchus populations to know something has to change, but what lacks is monitoring the factors contributing to their dismal fate. As of 2019, CITES began to monitor the commercial catch and trade of shortfin mako but agreed that it might not be enough (CITES, 2019). In 2013, MPI released a National Plan of Action for the Conservation and Management of Sharks (NPOA). In the plan, they stated one of their objectives was to “systematically review management categories and protection status to ensure they are appropriate to the status of individual shark species” (MPI, 2013, p. 19) among others, yet there is a lack of reporting available for public access to back this up. They also had intentions of issuing a revised NPOA in 2018, which seems not to exist altogether.

Bycatch records need to be scrutinised and fisheries must be forced to show accurate record-keeping. Data collected from accurate bycatch records of I. oxyrinchus can then be accumulated around the globe to highlight the fisheries that are exploiting shortfin makos.


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